The impact of photonics technology in the area of communication systems has been dramatic. In view of the advances, new communication system architectures have been proposed based on the photonics technology. The communication architectures take advantage of the ability of optical fibers to carry very large amounts of information with very little marginal cost once the optical fiber is in place.
With these architectures, switching operations are located at the edges of the network, large amounts of information are distributed throughout the network, and the information is filtered at gateways at the edges of the network. Further, these approaches often build on optical wavelength division multiplexing or optical frequency division multiplexing (coherent techniques) to deliver these large bundles of information throughout the network. The information carrying potential of the optical fiber systems is further enhanced with the use of external modulators for higher speed modulation of the guided light beam.
In the past, external modulators have been used to modulate the entire spectrum which is generated by the laser. Therefore, all of the wavelengths emitted by the laser carried the same information through the optical fiber. Since the different wavelengths travel at different speeds through the fiber, at large distances, interference between the wavelengths carrying the same information occurred.
Also in the past, multi-mode fibers have been utilized to carry different wavelength signals. However, different lasers have been utilized to generate the signals. Due in part to the ability to control the output wavelength of the lasers, the wavelengths have been selected to utilize two or more of the fiber optic windows of absorption. However, since a single window is typically the optimum transmission range, these devices suffer a drawback of not transmitting all information at the optimum wavelength window of absorption for the optical fiber over which the information will be transmitted.
Therefore, there arises a need for a photonic method and apparatus which enables external modulation of two or more longitudinal modes of a multi-longitudinal mode semiconductor laser in which each of the longitudinal modes can be independently and simultaneously modulated. By providing this combination of wavelength division multiplexing and external modulation, increased optical fiber information transmission capacity results. The present invention directly addresses and overcomes the shortcomings of the prior art.